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Q151. Does null and empty value for a column in Table means same ?

Ans. No.

Null value means that the value is yet to be assigned or need not be assigned whereas empty value means that the blank value has already been assigned once. Null is the default for the null able columns that hasn't be assigned any value.

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Q152. Where the prepared Statement object creating - In Java application or DB ?

Database

Ans. Prepared Statement objects are created and cached at database level.

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Q153. What are the benefits of using prepared statements ? Does that benefit extend only if similar prepared statements are used within a application or it extends even with multiple applications having similar prepared statement ?

Database

Ans. Prepared Statements helps in reuse of database execution plan which can be cached at Database level and hence helps achieving faster execution and performance. As cached ps objects are creating at Database level and not application level, the use and benefit extends across multiple applications.

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Q154. What are the benefits of using Prepared statements ?

Database

Ans. 1. Precompilation , DB-side caching and reuse of same execution plan leads to overall faster execution.

2. Automatic prevention of SQL injection attacks by escaping special characters

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Q155. Difference between log4j and slf4j ?

Logging

^{This question was recently asked at 'Y Media Labs'.This question is still unanswered. Can you please provide an answer.}

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Q156. Can we override an abstract method with abstract method in the derived class ?

Core Java

Ans. Yes, but in that case the derived class itself should be abstract. We cannot have an object of a class without definition for the method which has been declared abstract in the parent.

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Q157. Can we override abstract methods ?

Core Java

Ans. Abstract methods are the methods that need to be overridden in the derived class ( either as implementing method or again abstract method ) so it's not only allowed but its required to override abstract method in the derived class.

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Q158. Can we overload abstract methods ?

Core Java

Ans. Yes

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Q159. Can the foreign key column have null values ?

Database

^{This question was recently asked at 'Volkswagen It services'.This question is still unanswered. Can you please provide an answer.}

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Q160. write program to do matrix multiplication

Core Java

^{This question was recently asked at 'Kony Labs'.This question is still unanswered. Can you please provide an answer.}

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Q161. Write program to create a linked list and perform different operations on it.

Algorithm

Ans. import java.util.*;
class LinkedListSolution{
protected LinkedList list;
public LinkedListSolution(){
list = new LinkedList();
}
public Object pop() throws NoSuchElementException{
if(list.isEmpty())
throw new NoSuchElementException();
else
return list.removeFirst();
}
public void push(Object obj){
list.addFirst(obj);
}
public Object peek() throws NoSuchElementException{
if(list.isEmpty())
throw new NoSuchElementException();
else
return list.getFirst();
}
public boolean isEmpty(){
return list.isEmpty();
}
public String toString(){
return list.toString();
}
}
class TestStack{
public static void main(String args[]){
LinkedListSolution s = new LinkedListSolution();
s.push("First");
s.push("Second");
s.push("Third");
System.out.println("Top: " s.peek());
s.push("Fourth");
while(!(s.isEmpty()))
System.out.println(s.pop());
}
}

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Q162. What are the core OOPs concepts ?

Core Java

Ans. Abstraction, Encapsulation, Polymorphism , Composition and Inheritance

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Q163. What are the methods to connect to database in Java ?

Database

Ans. JDBC ( Java Database Connectivity ),ODBC (Open Database Connectivity), Hibernate, JPA ( Java Persistence API ), JOOQ,MyBatis

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Q164. Difference between Singleton and Factory Design Pattern ?

Design

Ans. Both are creational design patterns but singleton facilitates in creation and reuse of single object whereas Factory deals with creation of multiple objects.

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Q165. What is a Webdriver ?

Testing

Ans. Selenium WebDriver is a tool for automating web application testing.It helps in replicating the manual tester behavior like keyboard entry, mouse events etc and then matching the output against the expected.

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Q166. isnt the use of HashTable and ConcurrentHashMap the same, i.e providing synchronized map collection ?

Core Java

Ans. Yes, they both aim at providing synchronized access to the Map collection. The only difference is in their implementation. ConcurrentHashMap came after HashTable and hence technically more efficient as it doesn't lock the complete map while accessing it.

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Q167. Difference between concurrentHashMap and HashTable ?

Core Java

Ans. HashTable locks the complete collection to provide synchronization whereas ConcurrentHashMap only locks segments to achieve synchronization and hence better efficient and faster.

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Q168. Create a Table with 2 Columns and Insert a row into it

Database

Ans. CREATE TABLE TABLE_NAME (
ID NUMBER PRIMARY KEY,
NAME VARCHAR(50) NOT NULL,
);

INSERT INTO TABLE_NAME(ID, NAME) VALUES(1, "Abc");

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Q169. What are the benefits of creating immutable objects ?

Core Java

Ans. 1. Security and Safety - They can be shared across multiple threads as they are thread safe. Moreover, it protects then from bad state due to interception by the other code segment. One such problem due to mutability and access by alternate code segment could be the change of hash code and then the impact on its search with hash collections.

2. Reuse - In some cases they can be reused as only one copy would exist and hence it can be relied upon. For example - String Pool

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Q170. How should we handle errors while writing or accessing Stored Procedures?

Database

Ans. Store Procedure returns the error code. Moreover we can put the call withing try block and catch SQL Exception.

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Q171. What actions you take if there is an issue related to Database server ?

Support

Ans. We involve DBA and try to solve it through them. By the time they are solving it , we keep the stake holders informed regarding the progress.

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Q172. Which interface is responsible for transaction management in JDBC ?

Database

Ans. Connection Interface

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Q173. Which environment variables do we need to set in order to run Java programs?

Core Java

Ans. PATH, CLASSPATH and JAVA_HOME

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Very frequently asked. Usually asked in this format or as difference with interfaces / concrete classes.

Q174. What is an abstract class ?

Core Java

Ans. Abstract class is the class that is not supposed to be instantiated. The purpose of the class to only have extension to the derived class.

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Q175. What is a factory design pattern ?

Design

Ans. Factory Design Patterns is the pattern that recommends creation of separate Factory Object for creation of other object.

So its like saying - If you want to create an object of ClassA, Talk to FactoryObject ( which is an object of FactoryClass ). FactoryObject in itself encapsulates the inputs and logic required to make the decision regarding the creation and construction of object.

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Q176. What are Inner , Outer , Left and Right Joins in SQL ?

Database

Ans. Inner join is the intersection of two tables on the condition defined by the where clause i.e will get records from both tables matched by a column.

Outer join is the union of two tables i.e will get all records from both tables and will put null in the columns where related records are not present.

Left Outer join is the left union of two tables i.e all records from the table on the left and values from the right table for related records else null for the columns from right table.

Right Outer join is the right union of two tables i.e all records from the table on the right and values from the left table for related records else null for the columns from left table.

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Q177. What is database Normalization ?

Database

Ans. https://en.wikipedia.org/wiki/Database_normalization

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Q178. What are different types of variables a class can have ?

Core Java

Ans. 1. Method local variables - These are declared and defined within a method ( instance or static methods ) and their scope is limited to the method itself. They are destructed once the execution of method completes. They are stored in stack memory.

2. Instance variables - These are declared as non static variables as part of the class.They are initialized as part of object creation ( constructor ) and are destructed by java's garbage collection mechanism and hence stored in heap.

3. Static variables - These are declared with the static keyword and are part of the class. They are initialized at the time of class loading and are destructed by java's garbage collection mechanism and hence stored in heap.

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Q179. When should we use Abstract classes ?

Core Java

Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as inheritance ( extending abstract class ). So they should be used in place of interfaces in case there is some code ( methods ) or object body ( member elements ) that can be reused with inheritance.

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Q180. With a Table employee having columns Id,Name,ManagerEmployeeId, Write a SQL to find the Manager with maximum number of employees ?

Database

Ans. Select Name from EMPLOYEE where ID in (Select ManagerEmployeeId from EMPLOYEE
Group By ManagerEmployeeId
order by count(Id)
LIMIT 1)

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